The present invention relates to metallic corrosion protective coatings of iron and iron-based materials, in general, and in particular to zinc-based diffusion coatings of such materials, and to methods of producing such diffusion coatings.
It is known that metallic sacrificial corrosion-protection coatings for iron-based materials may be categorized into two main groups: thick metallic coatings for long-term outdoor applications, and thin metal coatings for limited-term outdoor applications or for indoor applications. These coatings are used to coat various surfaces, typically mechanical components such as nails, washers, bolts, screws, nuts, chain links, springs and the like.
The most popular technology for the thick coatings category is zinc hot-dip coating, also known as zinc galvanizing. In this technology, an iron or steel substrate is coated with a zinc layer, by passing the substrate through a molten bath of zinc at a temperature of around 460° C. Modern types of these coatings additionally contain aluminum, magnesium and silicon (see, by way of example, Y. Morimoto et al., “Excellent Corrosion-resistant Zn—Al—Mg—Si Alloy Hot-dip Galvanized Steel Sheet “SUPER DYMA”, Nippon Steel Technical Report No. 87, January 2003). The thickness of coatings obtained by this technology usually varies between 40 μm and 100 μm.
Metallic coatings of the thin zinc-based coatings category are generally useful, as already mentioned, for indoor applications and for limited outdoor applications. These coatings are typically used as a base for organic and inorganic topcoats that provide additional required attributes like improved corrosion protection, hardness, color, etc.
The thickness of coatings of this group is usually between 4 μm and 15 μm. However, such a thickness generally provides, in and of itself, insufficient corrosion protection, and additional protection, such as a chromate passivation layer, or sealing with organic or inorganic sealers, is necessary.
The main industrial method of zinc thin coatings production is electrodeposition, also known as electroplating. This process is analogous to a reversed galvanic cell. The part to be plated is the cathode of an electric circuit, while the anode is made of zinc. Both components are immersed in an electrolyte containing one or more dissolved metal salts, such as nickel, cobalt, and manganese, as well as other ions that permit the flow of electricity. A rectifier supplies a direct current to the cathode causing the metal ions in the electrolyte solution to lose their charge and plate out on the cathode. As the electrical current flows through the circuit, the anode slowly dissolves and replenishes the ions in the bath.
Various polymetal zinc-based alloy coatings such as zinc-nickel, zinc-cobalt, zinc-iron and zinc-manganese coatings are also widely manufactured. However, it is very difficult to obtain zinc-based alloy coatings, and polymetal zinc-based alloy coatings in particular, that have a substantially uniform thickness. Usually, these coatings have some non-coated areas, as well as a highly non-uniform thickness.
As used herein in the background of the invention, the specifications and the claims section that follows the term “uniform coating” and the like, refers to a zinc diffusion coating where the deviation of individual measurements of the coating thickness are smaller than 20% of the average thickness; and the term “continuous coating” refers to a zinc diffusion coating where the coating layer coats at least 95% of the surface of the iron-based substrate.
Medium-thickness corrosion-protective coatings, of between 15 μm and 50 μm, are produced by the above-mentioned electrodeposition method, and by an additional method known as diffusion coating, vapor galvanizing, or Sherardizing. According to this method, a layer of zinc is applied to the metal substrate by heating the substrate in an airtight container containing zinc powder.
It should be stressed that Sherardizing is ideal for coating small parts, and inner surfaces of small components, as frequently required by many industries, such as the automotive industry.
In this zinc diffusion coating process, the zinc diffusion coatings are actually zinc-iron intermetallic diffusion layers of iron-based substrates. The basic concept of the process is simple: parts coated with powder mixtures containing zinc powder are loaded into a special sealed vessel, and heated up to temperatures of 340° C. to 450° C. In this temperature range, zinc atoms diffuse into the substrate and a zinc-iron intermetallic diffusion layer is formed. The thickness of the diffusion layer is a function of the process temperature, dwelling time and the quantity of the zinc powder.
It is also known that the European specification EN 13811-2003 divides zinc diffusion coatings into three classes according to their thickness range: Class 15 for coatings equal to, or greater than 15 μm, Class 30 for coatings equal to, or greater than 30 μm, and Class 45 for coatings equal to, or greater than 45 μm.
It should be noted that zinc diffusion coatings thinner than 15 μm are not characterized by these specifications because to date, such coatings have been prone to damage, do not completely cover the surface of the substrate, and are highly non-uniform. Therefore, zinc diffusion coatings thinner than 15 μm do not generally provide the required corrosion protection or the additional demanded attributes to the coated parts, and, hence, have not been widely applied in industry.
There is therefore a recognized need for, and it would be highly advantageous to have thin, continuous and uniform zinc-based diffusion coatings on iron-based materials, and methods of producing the coatings. Such thin, continuous and uniform zinc-based diffusion coatings may provide good corrosion protection to iron-based parts and serve as an excellent base for additional coatings. It would be of further advantage for the methods of producing such coatings to be simple, cost effective, and environmentally friendly, with respect to known methods.